History of Three Mile Island Three Mile Island was one of the worst accidents at a nuclear power plant. This occurred on March 28, 1979 on an island near Harrisburg, Pa. Half the fuel was melted in one of the 2 nuclear reactors, large quantities of radioactivity leaked but a lot of it was enclosed. There were no harmful amounts of radiation towards the people. The damage was revealed years later when special ultrasonic, sonar-like systems looked inside the reactor vessel. Cause The accident happened at 4 am when there was a malfunction in the second cooling circuit which eventually caused the temperature in the coolant to rise. The reactor soon shut down automatically and only took about one second but a relief valve had failed to close but nothing revealed this had happened. A lot of the primary coolant drained away, and the heat decay in the reactor core was not removed and received severe damage. The operators were unable to respond properly to the unplanned shutdown of the reactor. The control room instrumentation and inadequate emergency response training proved to be the cause of the unplanned accident.

Results: As a result to the accident it brought many changes involving emergency response planning, reactor operating training, human factors engineering, radiation protection, and many other power plant operations. It cause the U.S. Nuclear Regulatory Commission to expand its regulatory oversight the radioactivity was released outside but not enough to do any damage to local residents.